The present invention relates to a radio receiver, particularly to a receiver for use in fixed-frequency applications such as GPS, and to a frequency generator, which may be used in such a radio receiver, or elsewhere.
GPS receivers have until now conventionally used expensive temperature controlled crystal oscillators, in order to achieve the required accuracy in frequency generation. The present invention seeks to provide a system which permits a less expensive method of frequency generation, and also to provide a particularly economic method of frequency generation, without using a temperature controlled crystal oscillator, voltage controlled oscillator, phase-locked oscillator or synthesizer. This approach results in greatly enhanced phase noise performance.
According to a first aspect of the present invention, there is provided a frequency generator, comprising: a single transistor oscillator arranged such that a signal of a basic frequency appears at a first output terminal of the transistor; and at least one reactive circuit element, having an impedance which increases with increasing frequency throughout a frequency range of interest, the reactive circuit element being connected to the same transistor such that a signal of a second frequency appears at a second output terminal of the same transistor, the second frequency being within the said frequency range, and being a multiple of the basic frequency.
Advantageously, the basic and second frequencies may be widely spaced, for example by a factor of about 20, such that the second frequency may be in the microwave range, while the basic frequency may be useable, directly or after division, as a system reference frequency.
Preferably, the frequency generator comprises a tuned circuit between the emitter of the transistor and ground, such that a voltage signal at the basic frequency appears on the emitter terminal of the transistor. Preferably the reactive element is connected between the collector terminal of the transistor and a power supply rail. Preferably the reactive element is chosen such that the signal appearing on the collector terminal of the transistor include higher order multiples of the basic frequency, for example twenty times the basic frequency.
Preferably the reactive element is chosen such that it has a high impedance in the region of the second frequency.
This arrangement has the advantage that the two frequencies appear on separate ports. The basic frequency needs no further filtering, while the harmonic frequency appearing on the collector terminal includes only very small components of the basic frequency. Further filtering is then needed only to remove adjacent harmonics which will also appear.
According to a second aspect of the present invention, there is provided a radio receiver including radio receiver circuitry for connection to digital signal processing circuitry, and comprising a single transistor oscillator arranged such that a signal of a basic frequency appears at a first output terminal of the transistor; and at least one reactive circuit element, having an impedance which increases with increasing frequency throughout a frequency range of interest, the reactive circuit element being connected to the same transistor such that a signal of a second frequency appears at a second output terminal of the same transistor, the second frequency being within the said frequency range, and being a multiple of the basic frequency, wherein the second frequency is used as a local oscillator frequency for the radio receiver circuitry and the basic frequency signal is used as a clock frequency for the signal processor.
Clearly, this has the advantage that it reduces the complexity of the overall circuit.
According to a third aspect of the present invention, there is provided a radio receiver, comprising a single transistor oscillator arranged such that a signal of a basic frequency appears at a first output terminal of the transistor; and at least one reactive circuit element, having an impedance which increases with increasing frequency throughout a frequency range of interest, the reactive circuit element being connected to the same transistor such that a signal of a second frequency appears at a second output terminal of the same transistor, the second frequency being within the said frequency range, and being a multiple of the basic frequency, wherein the second frequency signal is used as a first local oscillator signal and the basic frequency signal is used as a second local oscillator signal.
According to a fourth aspect of the present invention, there is provided a radio receiver, including radio receiver circuitry having means for generating an oscillator signal, and signal processing means, characterized by the use of an uncompensated oscillator for generating the oscillator signal, whereby the signal processing means tolerates the resulting frequency errors.
By the term "uncompensated" is meant that the circuit is not compensated against variations due to temperature, tolerances, or ageing. Thus, there is no temperature sensing, or means of applying an active control to the tuned circuits.
Preferably, the final mixer in the receiver circuitry is either software controlled hardware or implemented wholly in software to remove any residual offset.
Thus, frequency errors in the oscillator signal may be of the order of tens of parts per million, rather than a few parts per million, without any deterioration in the final output.
According to a fifth aspect of the present invention, there is provided a fixed frequency dual downconversion receiver, comprising: an analogue first downconversion and IF filter stage; means for one-bit coding of the downconverted signal; and means for second downconversion by digital subsampling.
According to a sixth aspect of the present invention, there is provided a fixed frequency dual downconversion radio receiver, having: an analog first downconversion and IF filter stage; means for one-bit coding of the downconverted signal; means for second downconversion by digital subsampling; means for generating a microwave local oscillator signal comprising an uncompensated crystal and a single transistor, the microwave local oscillator signal appearing on one terminal of the transistor, and a reference frequency appearing on another terminal of the transistors; and a digital signal processor, having a reference frequency (derived directly or by division from the basic frequency) as its clock signal, and comprising means for tolerating frequency errors arising from the use of the uncompensated crystal in the frequency generator.
According to a seventh aspect of the present invention, there is provided a method of processing a received radio signal in a receiver, the method comprising: downconverting the signal to an IF; filtering the downconverted signal; one-bit coding the filtered downconverted signal; and digitally subsampling the coded signal.